1. Field of the Invention
The present invention relates to a thermionic emission cathode using, as cathodes, materials of calcium hexaboride type such as lanthanum hexaboride for emitting electrons, and more particularly to a thermionic cathode using highly oriented (anisotropic) carbonaceous materials which tightly hold a cathode emission tip between suitable electroconductive members and at the same time supply thermal energy sufficient for emission of electrons as a heater.
The term "tip supporting heater" used hereafter refers to a member comprising such carbonaceous materials which supports or tightly holds the cathode emission tip between electroconductive members and concurrently heats the emission tip to working temperature
2. Brief Description of the Prior Art
Rare earth borides, in particular lanthanum hexaboride, have properties which make it highly suitable as a cathode material. The characteristic which has prevented the wide-range use of lanthanum hexaboride as an emitter material is its highly reactive nature at high operating temperatures. Most supporting members will react with lanthanum hexaboride to cause deterioration of the supporting members. Therefore, the lifetime of such cathodes is disadvantageously short. In order to eliminate this disadvantage, various proposals have heretofore been made regarding thermionic structures of this type.
U.S. Pat. No. 4,054,946 discloses a cathode device which comprises holding a single crystal LaB.sub.6 tip with two pieces of vitreous carbon and supporting the pieces by molybdenum jaws. However, vitreous carbon or glassy carbon materials are extremely rigid and prone to break into glassy fragments so that it is difficult to freely process them into a desired shape and size.
Further, U.S. Pat. No. 4,068,145 discloses a device comprising a heating member using pyrolytic graphite or boron carbide as a means for heating an emitter tip which is designed so as to nullify an undesired shift of the emitter due to thermal deformation by supporting the heating member with elastic electroconductive members. However, this device consumes a considerable amount of an electric power when the heater is in a heated condition and temperature changes are liable to occur even though a constant current is continuously supplied. Therefore, stability is not satisfactory. Furthermore, the device becomes unavoidably large in scale and thus it was impossible to apply this device to conventional tungstenmade hairpin cathode devices.
The present inventors have found a novel carbonaceous material such as lanthanum hexaboride which is chemically active at high working temperatures and suitable for supporting and simultaneously heating an emitter tip.